March 2012

Functions of cardiolipin as modifiers of the Barth syndrome phenotype


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Barth syndrome, or BTHS, is an X-linked genetic disorder characterized by dilated cardiomyopathy, skeletal myopathy, neutropenia and 3-methylglutaconic aciduria, resulting from mutations in the tafazzin gene (1). Tafazzin is a transacylase that remodels the mitochondrial phospholipid cardiolipin. Key findings linking CL to BTHS include the fact that BTHS fibroblasts exhibit decreased CL, defective acylation of CL with unsaturated fatty acids, and accumulation of the intermediate monolysocardiolipin. Importantly, the predominant CL species in the heart, tetralinoleoyl-CL, is absent from BTHS cells. BTHS is the first disorder known to result directly from perturbation of CL metabolism. Like many monogenic disorders, BTHS is characterized by a wide range of symptoms, from severely debilitating and often fatal to nearly asymptomatic. The molecular mechanisms underlying clinical disparities in monogenic disorders are not understood, because most genes exhibit multiple interactions. Understanding these interactions will shed light on the disparities.

Elucidating functions of CL may identify modifiers of BTHS
Yeast has been pivotal in elucidating the functions of CL, as null mutants are available for every step of CL synthesis and genetic and genomic analyses readily can be applied (2). The pathway for CL synthesis is highly conserved. Yeast tafazzin mutants exhibit the same biochemical and respiratory deficiencies as BTHS cells and are complemented by the human tafazzin gene. Exciting findings in yeast indicated that CL plays an important role not only in mitochondrial bioenergetics but also in essential cellular functions not associated with respiration, including mitochondrial protein import, PKC signaling and cell integrity, vacuolar function and V-ATPase activity, cell division, and ceramide metabolism and longevity, among others (2 – 5). Decreases in these activities may thus exacerbate the phenotype associated with CL deficiency.

CL is required for optimal mitochondrial protein import, which may be affected in BTHS
CL mutants exhibit decreased mitochondrial protein import, synthetic interactions with mutants in outer mitochondrial membrane translocases, and defective assembly of outer membrane complexes (6, 7). Intriguingly, this defect also was observed in a BTHS lymphoblast cell line (7). This has implications for BTHS, as a BTHS-like illness known as dilated cardiomyopathy with ataxia syndrome, or DCMA, is caused by mutations in the gene DNAJC19/TIM14, the likely homolog of the yeast mitochondrial import gene Tim14 (2). The clinical similarities of DCMA and BTHS suggest that the defects in BTHS may be caused or exacerbated by defective mitochondrial protein import.

Author's note

While the focus of this article is on the role of cardiolipin in Barth syndrome, the subtext echoes the sentiment of Yusuf Hannun and Dan Raben regarding the current threat to basic research (1).

The financial dilemma scientists currently face is exacerbated by the skewing of funding in the direction of translational and targeted research at the expense of curiosity-driven science. Basic research in simple model systems such as yeast often faces even greater funding challenges, despite the fact that the yeast model system provides researchers with the ability to apply powerful genetic approaches to study fundamental cellular processes in a time- and cost-effective system. The knowledge gained in a simple model system such as yeast can then often be applied to higher eukaryotes.

Our understanding of the role of CL in BTHS has its roots in basic research, much of it in the yeast model. Decades of biochemical studies have characterized the interactions of CL with mitochondrial proteins. The diagnosis of BTHS is now facilitated by technological advances in lipid chemistry and lipidomics that were developed by curiosity-driven research. The identification of yeast CL null mutants provided powerful tools for in vivo studies of CL function from which we have learned that CL is essential for optimal mitochondrial bioenergetics and for assembly and stability of electron-transport chain supercomplexes. Respiratory and supercomplex defects that characterize BTHS can be understood in the context of these studies. The role of CL in the assembly and import of mitochondrial proteins was first shown in yeast CL mutants and has since been verified in BTHS lymphoblasts. A simple growth experiment in yeast mutants and subsequent synthetic lethality and suppressor analyses, which cannot be readily carried out in more complex organisms, have led to the discovery that CL plays an important role in essential cellular functions apart from respiration. These functions may hold the key to identifying modifiers of the BTHS phenotype.

In summary, while the link between CL and BTHS has been known for just over a decade, our understanding of this disorder and of many other human diseases would not have been possible without basic, curiosity-driven research (2).

 

References
1. Hanun, Y. and Raben, D. The Toxic Professor Syndrome (2011). ASBMB Today
2. Vance, D. E. (2006) FEBS Lett. 580, 5430 – 5435

 

Prospects for elucidating the physiological modifiers of BTHS are promising
Numerous advances in both basic and targeted research constitute a strong foundation for studies to elucidate the modifiers of BTHS:

  1. 1) A vast literature is available describing the biochemistry of CL since its identification by Mary Pangborn 70 years ago (8).
  2. 2) Yeast mutants are available for all genes in the CL pathway, facilitating the application of "the awesome power of yeast genetics."
  3. 3) BTHS models are characterized in yeast, Drosophila, zebrafish and mice (2, 9).
  4. 4) Advances in lipid chemistry have dramatically improved the analysis of CL species, which can now be identified in blood spots (10).
  5. 5) The Barth Syndrome Foundation (http://www.barthsyndrome.org) has a large repository of medical information collected from patient records. The tremendous contribution of this organization to research and the dissemination of information about BTHS cannot be overstated.

In conclusion, exploiting the yeast model to elucidate CL functions is expected to identify physiological modifiers of CL deficiency, which may shed light on the disparity of symptoms in BTHS patients and may provide targets for new treatments of BTHS and other disorders of CL metabolism.

References
  1. 1. Schlame, M., and Ren, M. (2006) Barth syndrome, a human disorder of cardiolipin metabolism. FEBS Lett. 580, 5450 – 5455.
  2. 2. Joshi, A. S., Zhou, J., Gohil, V. M., Chen, S., and Greenberg, M. L. (2009) Cellular functions of cardiolipin in yeast. Biochim. Biophys. Acta. 1793, 212 – 218.
  3. 3. Chen, S., Liu, D., Finley, R. L., Jr., and Greenberg, M. L. (2010) Loss of mitochondrial DNA in the yeast cardiolipin synthase crd1 mutant leads to up-regulation of the protein kinase Swe1p that regulates the G2/M transition. J. Biol. Chem. 285, 10397 – 10407.
  4. 4. Chen, S., Tarsio, M., Kane, P. M., and Greenberg, M. L. (2008) Cardiolipin mediates cross-talk between mitochondria and the vacuole. Mol. Biol. Cell 19, 5047 – 5058.
  5. 5. Zhou, J., Zhong, Q., Li, G., and Greenberg, M. L. (2009) Loss of cardiolipin leads to longevity defects that are alleviated by alterations in stress response signaling. J. Biol. Chem. 284, 18106 – 18114.
  6. 6. Jiang, F., Ryan, M. T., Schlame, M., Zhao, M., Gu, Z., Klingenberg, M., Pfanner, N., and Greenberg, M. L. (2000) Absence of cardiolipin in the crd1 null mutant results in decreased mitochondrial membrane potential and reduced mitochondrial function. J. Biol. Chem. 275, 22387 – 22394.
  7. 7. Gebert, N., Joshi, A. S., Kutik, S., Becker, T., McKenzie, M., Guan, X. L., Mooga, V. P., Stroud, D. A., Kulkarni, G., Wenk, M. R., Rehling, P., Meisinger, C., Ryan, M. T., Wiedemann, N., Greenberg, M. L., and Pfanner, N. (2009) Mitochondrial cardiolipin involved in outer-membrane protein biogenesis: implications for Barth syndrome. Curr. Biol. 19, 2133 – 2139.
  8. 8. Pangborn, M. C. (1942) Isolation and purification of a serologically active phospholipid from beef heart. J. Biol. Chem. 143, 247 – 256.
  9. 9. Acehan, D., Vaz, F., Houtkooper, R. H., James, J., Moore, V., Tokunaga, C., Kulik, W., Wansapura, J., Toth, M. J., Strauss, A., and Khuchua, Z. (2011) Cardiac and skeletal muscle defects in a mouse model of human Barth syndrome. J. Biol. Chem. 286, 899 – 908.
  10. 10. Kulik, W., van Lenthe, H., Stet, F. S., Houtkooper, R. H., Kemp, H., Stone, J. E., Steward, C. G., Wanders, R. J., and Vaz, F. M. (2008) Bloodspot assay using HPLC-tandem mass spectrometry for detection of Barth syndrome. Clin. Chem. 54, 371 – 378.




lipids_Miriam_Greenberg

Miriam L. Greenberg (mlgreen@sun.science.wayne.edu) is an associate dean for research at the Wayne State University College of Liberal Arts and Sciences and a professor in the department of biological sciences.


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